The proposed work is to identify alternative molecules to Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1) that allow transendothelial migration (TEM) of leukocytes in response to inflammation. First, anti-PECAM-1 reagents, such as antibodies and soluble PECAM fragments, effectively block most, but not all, leukocyte TEM in vitro and in vivo. Approximately 20 percent of neutrophils and monocytes still retain the ability to migrate. Furthermore, both PECAM-1 genetic knockout mice and transgenic mice constitutively expressing high concentrations of soluble mouse PECAM (functional knockouts) have normal leukocyte TEM responses in inflammatory models. These results show that at least one alternative pathway exists. The specific aims of this project are to identify the alternative adhesion molecules when PECAM-1 is present and functioning (Aim I), when PECAM-1 is genetically absent (Aim II), and when PECAM-1 is present but apparently non-functional (Aim III). Identification of these alternative pathways in this project will yield insights into developing more effective therapies for inflammatory and autoimmune disorders.